Mahjong to me feels faster paced, a bit more luck based, with higher highs and lower lows. Rummikub to me feels slower paced, less luck based. The scoring system in mahjong allows for way bigger differences in score than how I usually play Rummikub. Rummikub is easier to learn than mahjong.
I personally prefer mahjong, I'd say both are sufficiently different to be worth trying. If I want to play a game I'd play mahjong, if I want a physical game I can play at home with diverse people I'd pick rummikub.